Laser soldering has been carried out to date in what is essentially an early stage, a stage directed to proving feasibility. It has demonstrated utility over mechanical soldering techniques because it can reach inaccessible soldering assemblies, such as those recessed in a housing. These early attempts have been applied to the assembly of small electronic components mounted on conventional plastic (epoxy fiberglass impregnated) boards, which prevented heat conduction to the rest of the assembly. Very small diameter wires of conventional construction were soldered using relatively small solder pads dabbed onto easily accessible conductors of a circuit. These early feasibility attempts used very small powered laser beams (7-50 watts) and focused the beams on the solder pad to microsized diameters (0.005-0.010 inch) in order to effect melting. These early feasibility attempts are referenced in the following:
(1) F. Burns and C. Zyetz, "Laser Micro Soldering", Apollo Lasers, Inc. Report. PA1 (2) E. R. Goodrich, "Lasers in Electronics", Circuits Manufacturing, Vol. 21, No. 7, July 1981. PA1 (3) R. Saunders et al, "Lasers-Operation, Equipment, Application, and Design", prepared by Engineering Staff of Coherent Inc., McGraw-Hill, 1980. PA1 (4) T. Kujawa, "Laser Soldering Boosts Productivity", Lasers and Applications, September 1982. PA1 a is the Gaussian radius at 1/e.sup.2 point PA1 ln is logarithm PA1 T.sub.m is the melting temperature of the solder minus the specimen preheat temperature PA1 P is the laser beam power in watts PA1 A is the surface absorptivity of the solder at 10.6 microns PA1 R is the thermal resistance per unit area of the system PA1 t.sub.c is the critical time to bring the solder to the T.sub.m temperature (beam on-time is approximately t.sub.c /0.975) PA1 C is the heat capacity of the system.
Unfortunately, such early feasibility activities used processing techniques which would not enable an assembler to solder assemblies with the new emerging design constraints of automotive applications. For example, assemblies of sensitive electronic components may now employ larger sized electrical lead conductors (greater than 0.030 inch in width and in many cases flat or rectangular in cross section) which must be soldered with a higher strength to withstand varying thermal expansion forces experienced in the engine compartment of a road vehicle. The prior usage of low powered, neatly focused laser beams provided several obstacles to effective soldering of such emerging design assemblies. The laser beam, so controlled to a microsized diameter, would not melt a sufficient amount of the solder pad to effect a reflowed fillet along the entire bottom surface of a large, flat electrical lead wire, resulting in a weak joint. Reflowing solder about a small, round wire is considerably easier due to surface tension about a round surface. If the interfacing diameter of the laser beam were to be modified or expanded, the beam energy/unit surface area would not be sufficient to melt the solder and effect the reflowed joint.
In some of the emerging automotive electronic units to be soldered (such as an electronic ignition module mounted in an engine compartment) the solderable joints are supported on ceramic substrates used to conduct heat away during normal vehicle operation of the module, thereby preventing destruction of the printed circuit and soldered joints due to momentary high currents. However, such ceramic substrates act as a heat sink and facilitate rapid run-off of heat during soldering which inhibits proper thermal control of the laser diameter, and certainly makes it impossible to use low powered laser beams if proper soldering is to be effected. Increasing the laser beam power by itself is not an answer to this dilemma because a high powered beam, focused at a desired diameter, will heat the solder excessively, leaving little room for processing error and resulting in damage to adjacent sensitive electronic parts.
What is needed is a method of laser soldering which permits recessed soldering assemblies, supported on ceramic substrates, and employing larger sized conductors, to be effectively soldered with increasingly stronger joints and soldered within a wide range of processing parameters making the process easily usable by manufacturing personnel without tight limits.